Staying cool In hot weather koalas seek out and hug the coolest tree trunk they can find, new research has found.

According to ecophysiologist, Professor Andrew Krockenberger of James Cook University in Townsville, this behaviour halves the amount of water they need to drink in a heat wave.

"If you save 50 per cent of the water you might have had to use, you then don't have dehydrating effects of a heatwave," says Krockenberger. "For a koala this is not trivial."

Koalas are tree-dwelling animals that spend 18 to 20 hours a day sleeping.

In hot weather, they slump on the trunks or branches of certain trees, pressing the front of their body to the tree.

"Usually people look at that and think the koala's tired or 'stoned', but that's really not at all what's happening here," says Krockenberger.

"This is a koala that is putting itself in a position which minimises the water that it has to evaporate."

Front or back?

Most of a koala's body is covered in inch-thick insulating fur that not only keeps them warm but generally stops them losing heat.

Krockenberger originally expected that koalas would expose their fronts, which are covered with a less-insulating thinner white fur, to the air so they can lose heat.

In a new study published today in Biology Letters, however, he and colleagues found otherwise.

Krockenberger and colleagues took pictures of koalas in their environment with an infrared camera and found the tree trunks and branches being hugged by koalas were much cooler than the surrounding air, in some cases by more than 5°C.

It seems that by lying on their fronts on a cool tree koalas can transfer heat out of their body to the tree, says Krockenberger.

Water loss model

The researchers used the infrared photos to generate a sophisticated computer model that told them how much water the koala can lose in a heatwave.

Heatwaves can be deadly to koalas if they lose too much water. In general the only water available to them is in the leaves they eat, but these leaves can also be toxic to the koala if eaten to excess.

The researchers say cool tree trunks will become increasingly important for koalas as heatwaves are set to increase in the future.

The findings underscore the importance of trees to koalas for reasons other than food, Krockenberger says.

"In this particular case the coolest of the trees was an acacia, which is not a food tree."